In working with premature and sick infants it is very important that the desired body temperature be consistently maintained. This may also be true with certain older patients such as wet victims and those in shock whose circulation has been compromised.
A particular problem with infants and especially pre-term infants is that they will need to be transferred from a hospital lacking equipment and specialists to a hospital that can meet the infant's needs. It is during this transfer that it is critical to maintain consistent skin temperature. A premature child has a large surface-to-volume ratio and heat is lost in proportion to the surface area. Premature infants are especially vulnerable because they do not have the usual subcutaneous fat layer gained in the last month of pregnancy.
A conservative estimate of the number of premature infants who might require such specialized care is 22,000 which is the number born each year in the United States weighing less than 1500 grams. It is estimated that one-third of these may be transferred between hospitals and thus will encounter the body temperature problems discussed. If we consider larger infants and term babies, the number would be much greater and perhaps on the order of 100,000 infants per year.
Visual and hand access to the infant is important. The infant must be watched for changes in skin color, type of breathing, chest respiratory movement, vomiting and convulsions. The various invasive tubes must be watched for proper position and function. The endotracheal tube, the intravenous tube, the intraarterial tube, the stomach tube, the urinary catheter, etc. must all be accommodated and serviced. Attention to these items usually means increased exposure to the environmental temperature and increased body heat loss.
The current state-of-the-art includes several unsatisfactory approaches to dealing with this problem. An isolette may be used which is a plastic box supplied with heated air as a means of infant temperature maintenance. Heat loss is by radiation to the walls and by exposure to cool air. Access is limited to arm holes in the sides of the isolette, unless the lid on the box is raised. A transport isolette, which is a modified isolette, is self contained on wheels which includes a respirator, a battery pack, suction apparatus and monitors. The infant is accessed only from above through the raising of a hinged cover. Another approach to this problem is the use of a semitrailer for transport of one or more full sized neonatal intensive care units. The bed surface is about four feet high and the infant is heated by radiant heaters about three to four feet above the bed. The radiant heaters are ineffective as they may be easily blocked by the bodies of medical personnel or drapes or the like.
Known warming pads available have crude control systems that do not respond to changes in body temperature. None of them are thermostatically regulated to keep the patient's skin at a constant temperature. The electrothermal blanket in Charles U.S. Pat. No. 1,356,965 is such a heated blanket. A heating blanket is shown in the Endo U.S. Pat. No. 4,656,334 but the control merely senses the presence of a body under the blanket and turns the setting of the blanket from high to another lower preset temperature. This thermostat is not intended to regulate the body temperature of the occupant but simply keep the blanket from staying uncomfortably hot when the user goes to sleep without requiring the user to turn it down.